Minor API changes to Http2Adapter and friends to ease implementation.

PiperOrigin-RevId: 364796725
Change-Id: I4f27ede87f25f1b1c35d6f00c6e971dffd48971a
diff --git a/http2/adapter/http2_session.h b/http2/adapter/http2_session.h
index 831cf73..ddbed44 100644
--- a/http2/adapter/http2_session.h
+++ b/http2/adapter/http2_session.h
@@ -21,9 +21,9 @@
 
   virtual int Consume(Http2StreamId stream_id, size_t num_bytes) = 0;
 
-  virtual bool want_read() = 0;
-  virtual bool want_write() = 0;
-  virtual int GetRemoteWindowSize() = 0;
+  virtual bool want_read() const = 0;
+  virtual bool want_write() const = 0;
+  virtual int GetRemoteWindowSize() const = 0;
 };
 
 class Http2Options {